Florida high-school gunman believed to have been equipped with smoke grenades

A law-enforcement officer in Parkland, Florida, on Wednesday. Wilfredo Lee/AP

A Florida lawmaker said the gunman in the high-school shooting that killed at least 17 people on Wednesday had done "enormous preparation" for the act.

The gunman is believed to have been equipped with a gas mask and smoke grenades.

He is thought to have pulled the school's fire alarm in the hope of causing massive casualties.

A lawmaker said the gunman who killed at least 17 people at a Florida high school on Wednesday had done "enormous preparation" for the act and was equipped with a gas mask and smoke grenades.

Sen. Bill Nelson of Florida said law-enforcement officials had briefed him on Nikolas Cruz, the 19-year-old suspect in the attack on Marjory Stoneman Douglas High School in Parkland, Florida.

The gunman is believed to have intentionally set off fire alarms "so the kids would come pouring out of the classrooms into the hall," Nelson said on CNN.

"There the carnage began," Nelson said.

Cruz, a former student at the high school, was taken into custody without incident and was cooperating with law-enforcement officials. He is believed to have been equipped with an AR-15 and "countless magazines," the Broward County Sheriff's Office told ABC News.

After the shooting, Cruz left the school by blending in with other fleeing students, law-enforcement sources and eyewitnesses told ABC News.

Nelson said the FBI was leading the investigation.

"It is clear attack was designed & executed to maximize loss of life," Sen. Marco Rubio of Florida said on Twitter.